In the generation of electricity, a boiler produces steam at sufficient pressure and temperature to do work on (turn) a turbine. When the turbine shaft is turned, a generator is also turned and thereby produces electricity. As the steam turns the turbine, the energy is exhausted at various stages of the turbine. For this steam to leave the turbine, it must overcome atmospheric pressure (14.7 lb/in.sup.2). By installing a condenser at the outlet of the turbine, a vacuum is imparted on the turbine outlet. This means that the steam does not have to push against atmospheric pressure to leave the turbine. Therefore, less fuel will be used to get the same work done in the turbine.
The condenser accomplishes this feat by the process of condensing the steam around many small tubes through which cool water is flowing. This exchange of heat from the steam to the water condenses the steam to very pure water for reuse in the boiler. Since this condensing process reduces the steam volume on the order of 1/60 of what it was at the turbine outlet, a vacuum is formed. If the heat transfer from the steam to the cooling water is reduced, then additional fuel is required by the boiler to create enough energy to push the steam against the backpressure at the turbine outlet.
In once-through cooling systems, biological fouling of the condenser tubes is a major factor in reducing the required heat transfer. Of all the components of the steam generation cycle, condensers are the most frequent source of poor unit efficiency and poor unit availability. Therefore, condensers must be cleaned at various intervals which incurs significant costs for cleaning and costs of replacement power while that particular unit is offline. For a typical 600-MWe base loaded coal-fired unit, a manual condenser cleaning can cost $3,200 plus as much as $100,000 in replacement power.
Chemical oxidants, especially chlorine, have proven effective in controlling microfouling in the condenser tubes and are widely used by utilities. Existing Federal regulations have restricted the application of chlorine for biofouling control to minimize free residual chlorine (FRC) in cooling water discharges. Recently, concern over toxicity from FRC or its reaction products has resulted in proposed Federal regulations further restricting chlorine levels in effluents. The existing limit of 0.2 mg/l allowable FRC will be reduced to 0.2 mg/l total residual chlorine (TRC) in cooling water discharges from stations with once-through and recirculating cooling systems. These environmental restrictions will result in a decrease in the effectiveness of chlorine as a biofouling control method--even to the point of making chlorination essentially useless for adequate fouling control.
The presently utilized method of chlorination is to inject chlorine upstream of the condenser to the entire cooling waterflow for a specified duration. The chlorinated water flows through the condenser and then out to the discharge canal. The point just prior to entering the discharge canal is the point source for meeting EPA chlorinated water effluent limitations. Therefore, this point of compliance is a limiting factor on the chlorine concentration in the condenser. In addition, chlorinating the entire cooling waterflow at once is a limiting factor on the amount of chlorine fed to each tube.
Therefore, a better technology must be developed so that biofouling can be controlled and heat rate maintained through the addition of chlorine, and yet meet the present and proposed effluent guidelines for chlorination. The present invention addresses the development of a novel condenser chlorination technique utilizing the injection of chlorine at relatively high concentrations targeted sequentially at groups of tubes or sections of the condenser tube sheet at a time via a manifold. In this manner, the high concentration of chlorine through a section of the condenser will be diluted at the condenser outlet by mixing with the nonchlorinated waterflow through the other sections, thus meeting effluent guidelines. The effectiveness of targeted, high concentration chlorine injection is further increased by reducing the flow in the targeted tube, thus increasing the chlorine contact time with the biofilm layer.
The cost-benefit of this chlorination method is conservatively projected to be approximately $250,000 per unit per year in fuel costs alone. Other factors to be added to the cost-benefit are in regard to costs of manual cleanings, costs of replacement power, and costs of installing and operating dechlorination equipment.
From an environmental point of view, this chlorination method will allow power plants to discharge no measurable residual chlorine to water systems. In addition, while the effective concentration of chlorine fed to the condenser tubes is much higher than past practice, the actual poundage of chlorine fed to the cooling water and then discharged to the environment will be cut by approximately 75 percent.